High-purity silicon is generally produced in a multistage process starting from metallurgical silicon generally still comprising a relatively high proportion of impurities. To purify the metallurgical silicon it can, for example, be converted into a trihalogensilane such as trichlorosilane (SiHCl3) that is subsequently thermally decomposed to afford high-purity silicon. Such a procedure is known from DE 29 19 086 A1, for example. Alternatively, high-purity silicon may also be obtained by thermal decomposition of monosilane (SiH4) as is described in DE 33 11 650 A1, for example.
In recent years, obtaining highest-purity silicon by thermal decomposition of monosilane has increasingly come to the fore. Thus, for example, DE 10 2011 089 695 A1 and DE 10 2009 003 368 B3 disclose reactors into which monosilane may be injected and in which highly heated silicon rods on which the monosilane is decomposed are arranged. The silicon generated is deposited in metallic form on the surface of the silicon rods.
So that the deposition may be better controlled, it is customary to inject into reactors such as that described in DE 10 2009 003 368 B3, for example, a mixture of monosilane and a carrier gas such as hydrogen rather than pure monosilane. However, care must be taken to ensure that this gas mixture does not become excessively hot. Above a temperature of 400° C. there is a danger of decomposition of the monosilane occurring even in the gas phase which can result in intensified formation of undesired byproducts. To avoid this, the concentration of monosilane in the mixture is normally kept very low.
In practice, the temperature of the gas mixture inside a reactor is very difficult to control since large temperature gradients exist inside the reactor. These problems are further intensified when the amount of the gas mixture injected into the reactor is increased to deposit larger amounts of silicon. In modern reactors, the target throughput of monosilane-containing gas mixture may be more than 10000 Nm3 (standard cubic meters).
Such a high throughput can easily lead to turbulent flows inside the reactor that results in an undesirably efficient heat exchange between the gas mixture and the highly heated silicon rods occurring. This brings about the mentioned undesired increase in the temperature of the gas mixture. The silicon rods are also cooled. This results in increased energy consumption.
Such disadvantages have hitherto been accepted since turbulent flows are certainly also associated with positive effects. It is desired that silicon be deposited as evenly as possible on all regions of the mentioned highly heated silicon rods. Turbulizing the gas mixture injected into the reactors is regarded as conducive thereto.
It could therefore be helpful to provide a process for thermal decomposition of monosilane where even at a throughput of monosilane-containing gas mixture of more than 10000 Nm3 the known problems occur only to a comparatively small extent, if at all.